1. Field of the Invention
The present invention relates to a compound semiconductor switching device which is the core device of a compound semiconductor switch microwave monolithic integrated circuit (MMIC), and a method of fabricating the same. More particularly, the present invention relates to a transistor of a semiconductor device that has a low insertion loss, high isolation, and a fast switching speed and is suitable for a high-power low-distortion high-frequency control circuit, and a method of fabricating the same.
2. Discussion of Related Art
In general, many mobile communication devices such as cellular phones and wireless local area network (LAN) equipment use GHz-band microwaves, and a switching device for switching such high-frequency signals is used in a switching circuit of an antenna or in a transmitter/receiver switching circuit.
As such a switching device, a field effect transistor (FET), such as a high electron mobility transistor (HEMT) which is a compound semiconductor transistor or a metal-semiconductor field effect transistor (MESFET), is largely used because it has a good transmission characteristic, an excellent operating voltage characteristic, a low consuming current, a simple bias circuit and an easy implementation of multiport and integrated circuit in a high frequency band.
In addition, it is required to minimize an insertion loss and improve isolation and switching speed in a high-frequency switching circuit. Specifically, it is very important to design a high-power switching device having excellent linearity for a radio-frequency control circuit used in cellular and analog terminals.
In order to reduce the insertion loss, conventional techniques selected a method of lowering the resistance of a channel region of a transistor used for a switch circuit by designing the channel region such that its doping concentration or width is as increased as possible.
However, the conventional art has a problem in that capacitance due to a Schottky contact formed between a gate electrode and a channel area increases, and thus high-frequency input signals are leaked from the Schottky contact and the isolation is deteriorated.
To overcome this problem, conventionally, a shunt transistor is installed during a circuit designing process to improve the isolation. However, this method results in another problem of chip enlargement and thus the cost increase.
Therefore, in order to fabricate a high-power high-frequency control circuit having improved power handling capability using a low-power switching device, the conventional art uses a circuit design technique such as an impedance transformation technique, a stacked FET method, and an inductor/capacitor (LC) resonant circuit technique, or a device structure modification technique such as a squeezed-gate FET structure, two kinds of pinch-off voltage FET structure, and a multigate structure.
However, the circuit design technique has another drawback in that the chip size increases due to the transmission line of a λ/4 transformer, a plurality of FETs used therein, and an inductor and a capacitor added around the switching device, and thus the cost also increases. And, the conventional device structure modification technique, like the circuit design technique, has a problem of increased chip production cost due to an additional mask process and an increased distance between source and drain.